Laser scanning projectors constructed from microelectromechanical system (MEMS) components can be relatively small, and therefore implemented into easily portable devices such as picoprojectors. These laser scanning projectors can be used to display fixed or moving video images on a screen, wall, lens (in the case of a smartglass wearable), or user's skin (in the case of a smartwatch wearable). Since modern digital media is often in a high definition format, it is desirable for such laser scanning projectors to be capable of image display in high definition.
In general, MEMS laser scanning projectors function by optically combining red, green, and blue laser beams to form an RGB laser beam, and then directing the RGB laser beam to either a bi-axial mirror, or a set of two uni-axial mirrors working in tandem, with one of the axes being a fast axis and the other axis being a slow axis. Driving of slow axis movement of the mirror (whether it be a bi-axial mirror or a uni-axial mirror) is performed quasi-statically and linearly, typically at a low frequency of around 60 Hz.
This linear slow-axis movement drives the mirror from its minimal angle to its maximal angle in two phases. In a “trace” phase, the mirror is driven slowly from its minimal angle to its maximal angle linearly, while the RGB laser beam is directed so as to impinge upon the mirror. In a “retrace mode”, the mirror is driven quickly back from its maximal angle to its minimal angle linearly, while the RGB laser is modulated so that it is not impinging upon the mirror.
Shown via the lines of 8 of FIG. 1A is a graph of a driving signal for the mirror vs. time, when quasi-statically and linearly driven. As can be observed, in the trace phase 10A, the drive signal rises from a minimal amount to a maximal amount of 1 in 15 milliseconds, and in the retrace phase 10B, the signal drops from the maximal amount to zero in under two milliseconds. A second cycle is shown, with the trace phase 11A and retrace phase 11B.
In some cases, non-linear drive may be desired during the trace mode. With a non-linear drive during the trace mode, shown as the line 9 in FIG. 1A, the trace phase 10A is separated into three zones, namely a first projection zone 1A during which the RGB laser is directed so as to impinge upon the mirror, a dead zone 2A during which the RGB laser is modulated so that it is not impinging upon the mirror, and a second projection zone 3A during which the RGB laser is directed so as to impinge upon the mirror. Since the RGB laser is not to impinge upon the mirror in the dead zone 2A, movement of the mirror can be sped up during the dead zone as 2A compared to movement during the first and second projection zones 1A, 3A.
The fast transitions between the first projection zone and dead zone, and between the dead zone and second projection mode, introduce unwanted resonance into the mirror movement. This is shown in FIG. 1B, where the actual movement 15 of the mirror itself when driven by the non-linear drive signal 9 is shown compared to the non-linear drive signal 9. It can be observed that the actual movement 15 is comprised of oscillations or “ripples” that indicate unwanted resonant movement. Movement during the retrace phase can also contribute to ripple—in general, any drive modes of the mirror that are not smooth cause ripple.
Note that ripples are similarly present in the actual movement 12 of the mirror when driven by the linear drive signal 8. Prior art ripple suppression techniques are capable of attenuating the ripples present when the linear drive signal 8 is used, however, these prior art ripple suppression techniques do not function when the non-linear drive signal 9 is used.
Therefore, further development is required.